Dimetrodon was not a Dinosaur

Confusion over the Pelycosaur Dimetrodon

Although found in a lot dinosaur model sets, the sail-backed reptile known as Dimetrodon was not a dinosaur. Dimetrodon was a synapsid, a member of a group of reptiles that has a single opening (fenestra) just behind the eye socket in the skull. Dinosaurs, lizards, crocodiles, Aves (birds) and marine reptiles such as the Plesiosaurs were diapsids. Diapsid vertebrates have two holes on each side of the skull. Mammals are also synapsids, so Dimetrodon was actually more closely related to the mammal line than to the Dinosauria, although the term “mammal-like” reptile that is often applied to this genus is misleading.

A number of species have been described, one of the largest Dimetrodon grandis measured up to 3.5 metres in length and would have weighed about as much as an African lion (Panthero leo). Fossils have been found in North America and Europe with the last species of Dimetrodon becoming extinct around 250 million years ago.